Commercial scale production of saturated branched-chain fatty acids or alkyl esters thereof using starting materials from renewable sources is gaining enormous interest because of their favorable properties, including better biodegradability as compared to petroleum-based materials, lower toxicity, lower flammability due to their lower vapor pressures, lower melting points, and lower viscosity. These properties make such fatty acids an important feedstock for the production of lubricants, greases, emulsifiers, cosmetic products, surfactants, biodiesel, hydraulic fluids, and many more products. In the petrochemical industry, for example, branched-chain hydrocarbons are consumed for improved octane numbers. Environmental concerns over the use of petroleum-based materials in the lubricant industry have stimulated much research to find suitable alternative materials. In this regard, lubricating fluids derived from renewable fats and oils are of interest because of their purported advantages over petroleum-based materials (Hill, K., Pure Appl. Chem., 79: 1999-2011 (2007)).
Branching induces increased vapor pressures and decreased melting points for the hydrocarbons. Surfactants derived from branched-chain fatty acids show favorable physical properties, including a lower viscosity and improved handling, typically without detriment to intended performance characteristics. Commercial demands for fatty acid products with enhanced performance benefits including higher solubility, ease of handling, better hard water tolerance, improved oxidative stability, lower melting point, lower viscosity, and improved formulations induce enormous industrial interest in such branched-chain fatty acids. There are many commercial products in the market that are derived from renewable resources, such as polylactide polymers and 1,3-propanediol (important intermediates for polymer syntheses) that are derived from biomass sugars by fermentation and are cost-competitive with petroleum-based materials (Carole, T. M., et al., Applied Biochem. and Biotech., 113-116: 871-885 (2004)).
Vegetable oils are also promising candidates as replacements for petroleum-based materials since they have excellent lubricity properties. Although these oils themselves have some commercial use, it is limited due to the presence of double bonds within their fatty acid alkyl chains which lead to oxidative stability problems when used at high temperature. Over the past decades, numerous chemical methods including electrophilic, nucleophilic, oxidative, and metal-catalyzed reactions have been developed that convert the common fatty acids found in renewable fats and oils to novel oleochemical compounds that have improved and/or new properties over the starting fatty acids. For example, chemical processes for the modification of soy oil for use in greases, hydraulic and drilling fluids, and printing inks have been developed (Erhan, S. Z. and M. O. Bagby, J. Am. Oil Chem. Soc., 68(9): 635-638 (1991); Erhan, S. Z., et al., J. Am. Oil Chem. Soc., 69(3): 251-256 (1992); U.S. Pat. No. 5,713,990).
Fatty acids produced from the cleavage of fats and oils derived from renewable sources are typically straight hydrocarbon chains with an even number of carbons. Saturated branched-chain fatty acid isomers, are generally derived from unsaturated fats and oils as a mixture of mono-methyl branched-chain fatty acids. The hydrocarbon chain length generally ranges from 4 to 30 carbons with 12 to 24 carbons being most common. The degree of unsaturation and chain length of a fatty acid are dependent on the triglyceride source from which it is derived. Usually, fatty acids originated from fats have a lower degree of unsaturation than those derived from oils and when the double bonds exist they are more commonly in a cis isomeric configuration.
Existing methods for making saturated branched-chain fatty acids include using clay catalysts, such as bentonite and montmorillonite, and give primarily oligomeric byproducts such as dimers and trimers with much lower yields of the intended fatty acid. No process for reusing clay catalysts has been developed. Another known alternative approach of using metal (e.g., Na+, K+) cationic zeolite catalysts requires acid treatment for its activation which is more costly and less environmentally friendly than the presently disclosed methods (U.S. Pat. Nos. 8,748,641 and 9,115,076). The skeletal isomerization of unsaturated linear chain fatty acids to branched product was previously carried out over a number of acidic catalysts including sulfated zirconia (Hino, M., et al., Solid Super acid Catal., 101: 6439 (1979)); metal-promoted sulfated zirconia (Keogh, R. A., et al., Fuel, 78: 721 (1999); Hsu, C. Y., et al., J. Chem. Soc. Chem. Commun., 22: 1645 (1992); Tomishige, K., et al., Appl. Catal. A, 194: 383 (2000)); silica-supported phosphotungstic heteropolyacids (Tomishige 2000); and acidic alumina-supported noble metal bifunctional catalysts (Juszczyk, W., and Z. Karpinski, Appl. Catal. A, 67: 206 (2001)). Commercially, unsaturated branched-chain fatty acids are also being produced as a byproduct during the dimer acid production process using unsaturated linear chain fatty acids (Berman, L. U., et al. (Eds.), The General Characteristics of Dimer Acid, IN The Dimer Acids, Humko Sheffield Chemical, Memphis, 1975, p. 5). Several clays are also commonly used as catalysts in the acid dimerization process (U.S. Pat. Nos. 3,632,822; 3,732,263; and 6,187,903), where the yield of dimer/trimer acids and monomeric branched-chain fatty acids is 75% and below 20%, respectively. More recently, large-pore zeolites, such as faujasite, beta structure (pore size>6 Angstroms) and mesoporous zeolites (>15 Angstroms), have been used for skeletal isomerization of unsaturated fatty acids (U.S. Pat. Nos. 6,831,184 and 6,723,862; and Ha, L., et al., Applied Catalysis A: General, 356: 52 (2009)) with better yield of branched-chain fatty acids. In these procedures, high catalyst loading, synthesis of expensive in-house zeolite, regeneration of used zeolite, lower conversion, and suppression of dimer acid formation are still issues to be addressed.
There thus exists an industrial need for methods of producing higher yields of saturated branched-chain fatty acids having greater affordability, increased environmental friendliness and economic catalyst activation, and more efficient catalyst regeneration capabilities.